Stabilized recombinant expression plasmid vector in hafnia alvei and applications thereof

ABSTRACT

One aspect of the present disclosure relates to a stabilized recombinant expression plasmid vector comprising a polynucleotide encoding an antitoxin gene which expresses a polypeptide that neutralizes a polypeptide toxic to a host cell, the toxic polypeptide being expressed by a toxin gene in the host cell, and a polynucleotide encoding a polypeptide expression product, and the stabilized recombinant expression plasmid vector is derived from a  Hafnia alvei  autonomously replicable backbone plasmid. Other aspects of the present disclosure relate to a transformant transformed with the stabilized recombinant expression plasmid vector disclosed herein, a method of producing biobased cadaverine using the transformant disclosed herein, and biobased cadaverine prepared by the method disclosed herein. Another aspect of the present disclosure relates to a polyamide formed using biobased cadaverine disclosed herein, and a composition thereof. Another aspect of the present disclosure relates to a method of preparing 1,5-diisocyanatopentane comprising preparing biobased cadaverine using the method disclosed herein and converting the biobased cadaverine to 1,5-diisocyanatopentane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 61/649,719, filed May 21, 2012, which is incorporated by reference as if fully set forth herein.

BACKGROUND

Cadaverine is a platform chemical involved in the production of various products. Bio-based production of cadaverine has gained research interest since the 1980s. Cadaverine can be synthesized via decarboxylation of lysine in microorganisms. Currently, biosynthesis of cadaverine is performed using two strategies: fermentative production or in vitro enzyme catalysis.

In a fermentative production of L-lysine approach, a lysine decarboxylase gene is added to a lysine producing bacteria strain (e.g. Corynebacterium glutamicum and Escherichia coli (E. coli)) to extend the lysine biosynthesis pathway to a cadaverine biosynthesis pathway. However, the reported cadaverine yield is lower than the lysine yield for other Corynebacterium glutamicum strains lacking the lysine decarboxylase gene. Such low yield may be due to the toxicity of the cadaverine product to the producing bacterial strain.

Alternatively, bacteria can be engineered or induced to produce lysine decarboxylase for the in vitro enzyme catalysis. One strategy involves inducing expression of a chromosomally encoded lysine decarboxylase gene in an un-engineered Hafnia alvei (H. alvei) strain. However, the reported yield of the enzyme is low. Another strategy involves engineering recombinant strains. For example, Japanese companies (JP2009028045, U.S. Pat. No. 7,189,543, CN102056889) have reported the construction of E. coli recombinant strains that over-express lysine decarboxylase and utilize either whole cell or cell lysate for catalysis. However, expression of large amounts of polypeptides that are toxic to the host cell causes expression plasmid instability over serial passage. Antibiotics are required in the medium to ensure plasmid stability during the culture.

Use of antibiotics may cause development of antibiotic resistant bacteria, and maintains high levels of antibiotic resistant microorganisms in the environment. See, e.g. Martinez, “Environmental pollution by antibiotics and by antibiotic resistance determinants,” Environmental Pollution (2009), Vol. 157, Issue 11, 2893-2902. However, antibiotic resistant bacteria potentially pose health and/or environmental hazards. Thus, there remains a need for a more effective recombinant plasmid vector that can remain stable through multiple rounds of serial passage without antibiotic selection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the construction of the pPlac-cadA-abtabi recombinant expression plasmid vector as discussed in Example 1. a) cadA PCR product produced using the E. coli BL21 chromosome as a template; b) plasmid pMD18-T-cadA produced by ligation of the cadA PCR product to the pMD18-T vector, wherein the short lacZ fragment was located 5′ to the cadA gene; c) pPlac-cadA plasmid after deletion of the lacZ fragment; d) abtlabi PCR product with HindIII sites amplified using H. alvei pAlvB as a template, the PCR product was then ligated to a pMD18-T vector containing a HindIII restriction site; and e) pPlac-cadA-abtabi recombinant expression plasmid produced by HindIII digestion and subsequent ligation of fragments from pPlac-cadA and the pMD18-T vector containing the abt/abi fragment.

FIG. 2 shows the two possible constructions of pPlac-cadA-abtabi recombinant expression plasmid (Type I and Type II) produced according to the methods disclosed herein.

FIG. 3 shows recombinant strain JM109/pPlac-cadA colony growth on LB and LB/Amp plates after serial subculturing and serial dilution as discussed in Example 3.

FIG. 4 shows recombinant strain Ha/pPlac-cadA colony growth on LB and LB/Amp plates after serial subculturing and serial dilution as discussed in Example 3.

FIG. 5 shows recombinant strain Ha^(c)/pPlac-cadA-abtabi colony growth on LB and LB/Amp plates after serial subculturing and serial dilution as discussed in Example 3.

FIG. 6 shows stability of plasmid pPlac-cadA in cured H. alvei.

FIG. 7 shows stability of type I pPlac-cadA-abtabi plasmid in cured H. alvei.

DETAILED DESCRIPTION

The following description provides specific details for a thorough understanding of, and enabling description for, embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these details. In other instances, well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the disclosure.

The abbreviations used for the amino acids, peptides, base sequences, and nucleic acids in the present disclosure are based on the abbreviations specified in the IUPAC-IUB Communication on Biochemical Nomenclature, Eur. J. Biochem., 138: 9 (1984), “Guideline for Preparing Specifications Including Base Sequences and Amino Acid Sequences” (United States Patent and Trademark Office), and those commonly used in this technical field.

A “nucleotide sequence,” “polynucleotide” or “DNA molecule” as contemplated by the current disclosure, may include double strand DNA or single strand DNA (i.e., a sense chain and an antisense chain constituting the double strand DNA), and a fragment thereof. As used herein, “a fragment thereof” means a part of the nucleotide sequence that encodes a peptide which provides substantially the same function as the peptide encoded by the whole nucleotide sequence. For example, a polynucleotide encoding an antitoxin gene expresses a polypeptide that neutralizes a toxin polypeptide. A fragment of the polynucleotide encoding the antitoxin gene expresses a polypeptide that can neutralize the toxin polypeptide, which provides substantially the same function as the polypeptide encoded by the whole sequence of the polynucleotide encoding the antitoxin gene. Similarly, a fragment of a polynucleotide encoding a toxin gene expresses a polypeptide that is toxic to a cell substantially the same as the toxin polypeptide expressed by the whole sequence of the polynucleotide encoding the toxin gene.

Nucleotide sequences, polynucleotides or DNA molecules as used herein are not limited to the functional region, and may include at least one of an expression suppression region, a coding region, a leader sequence, an exon, an intron and an expression cassette (see, e.g. Papadakis et al., “Promoters and Control Elements: Designing Expression Cassettes for Gene Therapy,” Current Gene Therapy (2004), 4, 89-113). Further, examples of nucleotide sequences or polynucleotides may include RNA or DNA. A polypeptide containing a specific amino acid sequence and a polynucleotide containing a specific DNA sequence may include fragments, homologs, derivatives, and mutants of the polynucleotide. Examples of mutants of a nucleotide sequence or polynucleotide (such as mutant DNA), include naturally occurring allelic mutants; artificial mutants; and mutants having deletion, substitution, addition, and/or insertion. It should be understood that such mutants encode polypeptides having substantially the same function as the polypeptide encoded by the original non-mutated polynucleotide.

One aspect of the invention relates to a stabilized recombinant expression plasmid vector comprising:

a polynucleotide encoding an antitoxin gene which expresses a polypeptide that neutralizes a polypeptide toxic to a host cell, the toxic polypeptide being expressed by a polynucleotide encoding a toxin gene in the host cell,

a polynucleotide encoding a polypeptide expression product, wherein

the stabilized recombinant expression plasmid vector is derived from an autonomously replicable backbone plasmid of a host cell.

In certain embodiments, the toxin gene is chromosomally encoded in the genome of the host cell.

In certain embodiments, the stabilized recombinant expression plasmid vector further comprises the polynucleotide encoding the toxin gene.

In certain embodiments, the polynucleotide encoding the toxin gene and/or the polynucleotide encoding the antitoxin gene is recombinant.

In certain embodiments, one or more genes of the toxin gene, antitoxin gene and polypeptide expression product gene are further optimized using codon optimization technology to provide better expression of the corresponding polypeptides in the host cell. For example, an optimized toxin gene may comprise a DNA sequence optimized to provide a better expression of the toxin polypeptide compared to SEQ ID NO:1 or SEQ ID NO:3. In certain embodiments, the antitoxin gene comprises a DNA sequence further optimized to provide a better expression of the antitoxin polypeptide compared to SEQ ID NO:2 or SEQ ID NO:4. In certain embodiments, the polypeptide expression product gene comprises a DNA sequence further optimized to provide a better expression of the polypeptide expression product compared to SEQ ID NO:5 or SEQ ID NO:6.

Codon optimization is a technique to maximize the protein expression in a host cell by increasing the translational efficiency of gene of interest. DNA sequence of nucleotides of one species is optimized into DNA sequence of nucleotides of another species. A DNA sequence is broken into triplets (codons). The codons of low frequency of an amino acid are replaced with codons for the same amino acid but of high frequency in the host cell. Accordingly, the expression of the optimized DNA sequence is improved in the host cell. See, e.g. www.guptalab.org/shubhg/pdf/shubhra_codon.pdf for an overview of codon optimization technology, which is incorporated herein by reference in its entirety.

As used herein, a toxin/antitoxin gene pair has two genes, one is a toxin gene which expresses a polypeptide toxic to a host cell, and the other is an antitoxin gene which expresses a polypeptide that neutralizes the toxic polypeptide in the host cell.

Certain prokaryotes have one or more chromosomally encoded toxin genes. Certain prokaryotes contain endogenous plasmids that encode specific toxin/antitoxin gene pairs that play a role in maintenance of the genetic information and response to stress. (See, Wertz et al. “Chimeric nature of two plasmids of Hafnia alvei encoding the bacteriocins alveicins A and B.” Journal of Bacteriology, (2004) 186: 1598-1605.) In either case, as long as the cell has one or more plasmids comprising antitoxin gene, the toxin is neutralized by the antitoxin that is continuously expressed by one or more plasmids to keep the cells alive. In certain prokaryotes, the antitoxin protein degrades faster than the toxin protein. If the plasmid comprising the antitoxin gene is lost from the cell, the toxin protein will exist longer than the antitoxin protein in the cell and kill or inhibit the growth of the cell. Therefore, plasmid comprising the antitoxin or the toxin/antitoxin gene is preferably maintained to keep the host cell alive.

Examples of the toxin/antitoxin gene pair include, without limitation, abt/abi gene pair and aat/aai gene pair, and fragments thereof. In certain embodiments, the toxin gene comprises a DNA sequence of SEQ ID NO:1, or SEQ ID NO:3. In certain embodiments, the antitoxin gene comprises a DNA sequence of SEQ ID NO:2, or SEQ ID NO:4.

As used herein, the term “host cell” means a microorganism cell that can be transformed with a stabilized recombinant express plasmid vector. An example of a host cell includes, without limitation, Hafnia alvei (H. alvei).

In certain embodiments, the host cell is free of endogenous plasmid either in its native form or by removing any endogenous plasmid. The term “cure” as used herein means to remove endogenous plasmid from the host cell. The resulting endogenous plasmid-free host cell is referred to as a “cured” host cell.

In certain embodiments, the host cell may be selected from any of the H. alvei strains, for example, endogenous plasmid-free H. alvei strains, H. alvei strains having pAlvA plasmids and the cured strains thereof (pAlvA⁻ strains), and H. alvei strains having pAlvB plasmids and the cured strains thereof (pAlvB⁻ strains).

In certain embodiments, the host cell is an industrial strain suitable to be used in industrial-scale or large-scale production. For example, industrial strains may be cultivated in a fermenter. The scale of culture may range from hundreds of liters to millions of liters. On the contrary, a laboratory strain usually is cultivated in a few liters or less. In certain embodiments, an industrial strain may grow in a simpler or more economical medium than laboratory strains.

A polypeptide expression product is a polypeptide produced by a host cell. Examples of polypeptide expression products include, without limitation, any polypeptide expression product that can be produced by E. coli., e.g. enzymes such as decarboxylases, hydrolases, and phosphorylase. In one embodiment, the decarboxylase is amino acid decarboxylase, e.g. lysine decarboxylase, tyrosine decarboxylase, arginine decarboxylase, ornithine decarboxylase, and glutamate decarboxylase. In another embodiment, a polynucleotide encoding a lysine decarboxylase comprises a haldc gene, a cadA gene, or a fragment thereof. In another embodiment, the polynucleotide encoding a lysine decarboxylase comprises a DNA sequence of SEQ ID NO:5, or SEQ ID NO:6. In another embodiment, the hydrolase is a N-glycosidase or a O-glycosidase, examples include, without limitation, glucosidase, α-glucosidase, β-glucosidase, mannosidase, α-mannosidase, β-mannosidase, fructosidase, β-fructosidase, xylosidase, α-xylosidase, β-xylosidase, galactosidase, α-galactosidase, β-galactosidase, lactase, amylase, α-amylase, β-amylase, myrosinase, chitinase, sucrase, maltase, invertase, hyaluronidase, and neuraminidase. In another embodiment, a polynucleotide encoding a β-galactosidase comprises lacZ gene or a fragment thereof.

An autonomously replicable backbone plasmid of a host cell may be any plasmid that can replicate in the host cell. In one embodiment, the stabilized recombinant plasmid is derived from a backbone plasmid that can replicate in H. alvei. Examples of the backbone plasmids include, without limitation, backbone plasmids that can replicate in E. coli. strains, e.g. pUC (e.g. pUC18 and pUC19 plasmids), pBR322 and pACYC plasmids, and plasmids derived therefrom.

As used herein, a recombinant plasmid “derived from an autonomously replicable backbone plasmid of a host cell” means the recombinant plasmid is constructed by inserting one or more polynucleotides encoding an antitoxin gene, one or more polynucleotides encoding a toxin gene, and/or one or more polynucleotides encoding a polypeptide expression product described herein, and any combination thereof, into the autonomously replicable backbone plasmid of the host cell.

Another aspect of the present disclosure relates to a transformant obtained by transforming one or more stabilized recombinant plasmid vector disclosed herein into a host cell.

As used herein, a transformant is a host cell that has been altered by introducing one or more recombinant plasmid vectors in the host cell. In certain embodiments, the transformant is obtained by introducing a recombinant plasmid vector through transformation into a host cell displaying competence to the plasmid vector.

An antitoxin gene transformant or toxin/antitoxin gene pair transformant shows improved plasmid stability compared to the same host cell transformed by a recombinant plasmid vector that does not contain an antitoxin gene or a toxin/antitoxin gene pair.

In one embodiment, the host cell is an endogenous plasmid-free H. alvei strain. The endogenous plasmid-free H. alvei strain in its native form may be plasmid-free. Alternatively, the endogenous plasmid-free H. alvei strain is a cured H. alvei strain as described supra. The stabilized recombinant plasmid vector comprises one or more antitoxin genes selected from the group consisting of abi gene, aai gene and fragments thereof, and/or one or more toxin/antitoxin gene pairs selected from the group consisting of abt/abi gene pair and aat/aai gene pair, and fragments thereof.

Another aspect of the present disclosure relates to a method of producing cadaverine comprising:

1a) cultivating a transformant comprising a stabilized recombinant expression plasmid vector disclosed herein;

1b) producing cadaverine using the culture obtained from step 1a to decarboxylate lysine; and

1c) recovering cadaverine from the reaction obtained from step 1b.

As used herein, “using the culture obtained from step 1a” may comprise further processes of the culture obtained from step 1a. For example, using a buffer solution to dilute the culture; centrifuging the culture to collect the cells; resuspending the cells in a buffer solution; or lysing the cells into cell lysate; or/and purifying lysine decarboxylase from the cell lysate.

The transformant may be cultured using a medium containing carbon sources and non-carbon nutrient sources. Examples of carbon sources include, without limitation, sugar (e.g. carbohydrates such as glucose and fructose), oil and/or fat, fatty acid, and/or derivatives thereof. The oil and fat may contain saturated and/or unsaturated fatty acids having 10 or more carbon atoms, e.g. coconut oil, palm oil, palm kernel oil, and the like. The fatty acid may be a saturated and/or unsaturated fatty acid, e.g. hexanoic acid, octanoic acid, decanoic acid, lauric acid, oleic acid, palmitic acid, linoleic acid, linolenic acid, myristic acid, and the like. Examples of derivatives of a fatty acid include, without limitation, esters and salts thereof. Examples of non-carbon sources include, without limitation, nitrogen sources, inorganic salts, and other organic nutrient sources.

For example, a medium may contain a carbon source assimilable by the transformant, optionally with one or more other source selected from the group consisting of a nitrogen source, an inorganic salt and another organic nutrient source. In certain embodiments, the weight percentage of the nitrogen source is about 0.01 to 0.1% of the medium. Examples of the nitrogen source may comprise ammonia, ammonium salts (e.g. ammonium chloride, ammonium sulfate and ammonium phosphate), peptone, meat extract, yeast extract, and the like. Examples of the inorganic salts include, without limitation, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, magnesium phosphate, magnesium sulfate, sodium chloride, and the like. Examples of the other organic nutrient source include, without limitation, amino acids (e.g. glycine, alanine, serine, threonine and proline), vitamins (e.g. vitamin B1, vitamin B12 and vitamin C), and the like.

The culture may be carried out at any temperature as long as the cells can grow, and preferably at about 20 to about 40° C., or about 35° C. The culture period may be about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, or about 10 days.

In one embodiment, the transformant is cultured in a medium containing peptides, peptones, vitamins (e.g. B vitamins), trace elements (e.g. nitrogen, sulfur, magnesium), and minerals. Examples of such medium include, without limitation, commonly known Lysogeny broth (LB) mediums comprising tryptone, yeast extract and NaCl suspended in water (e.g. distilled or deionized).

In another embodiment, step 1c of the method further comprises the following steps:

1d) separating the solid and liquid components of the reaction obtained from step 1b;

1e) adjusting the pH of the liquid component obtained from step 1d to about 14 or higher;

1f) removing water from the liquid component obtained from step 1e; and

1g) recovering cadaverine.

In step 1d, the separation of the solid and liquid components of the reaction of step 1b may be accomplished by conventional centrifugation and/or filtration.

In step 1e, the pH of the liquid component of step 1d may be adjusted by adding a base, e.g. NaOH. NaOH may be added as a solid and/or a solution (e.g. an aqueous solution).

In step 1f, the water may be removed by distillation at ambient pressure or under vacuum.

In step 1g, cadaverine may be recovered by distillation at ambient pressure or under vacuum.

Another aspect of the present disclosure relates to biobased cadaverine prepared according to the method disclosed herein.

As used herein, a “biobased” compound means the compound is considered biobased under Standard ASTM D6866.

Another aspect of the present disclosure relates to a polyamide having a structure of Structure 1:

including stereoisomers thereof, wherein:

-   -   m=4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,         21, or 22;     -   n=4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,         21, or 22;     -   j=about 100˜about 1,000,000; and     -   the polyamide is prepared from one or more diamines having         carbon numbers of m and one or more dicarboxylic acids having         carbon numbers of n, at least one of the diamines and         dicarboxylic acids comprises biobased carbon under Standard ASTM         D6866, and the m or n of each diamine or dicarboxylic acid can         be the same or different.

In one embodiment, the diamine is biobased cadaverine, more preferably biobased cadaverine prepared according to the method disclosed herein. Examples of the dicarboxylic acids include, without limitation, C₁₀dicarboxylic acid, C₁₁dicarboxylic acid, C₁₂dicarboxylic acid, C₁₃dicarboxylic acid, C₁₄dicarboxylic acid, C₁₆dicarboxylic acid, C₁₈dicarboxylic acid, and any combinations thereof. In certain embodiments, all or part of the C_(n)dicarboxylic acids are biobased.

In another embodiments, the polyamide has a structure described above, wherein:

-   -   the polyamide is formed by reacting biobased cadaverine with one         or more dicarboxylic acids, more preferably the biobased         cadaverine is prepared according to the method disclosed herein.     -   n=4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,         21, or 22;     -   j=about 100˜about 1,000,000, about 1000˜about 100,000, or about         1000˜about 10,000; and     -   the dicarboxylic acids comprise biobased carbon under Standard         ASTM D6866.

Another aspect of the present disclosure relates to a method of making the polyamides disclosed herein comprising

-   -   preparing biobased cadaverine as the C_(m)diamine according to         the method disclosed herein.

In one embodiment, the method further comprises preparing one or more biobased C_(n)dicarboxylic acids.

In another embodiment, the method further comprises preparing the polyamide by reacting biobased cadaverine with one or more biobased C_(n)dicarboxylic acids.

Another aspect of the present disclosure relates to a composition comprising one or more polyamides disclosed herein.

In one embodiment, the diamine is biobased cadaverine, more preferably biobased cadaverine prepared according to the method disclosed herein. Examples of the dicarboxylic acids include, without limitation, C₁₀dicarboxylic acid, C₁₁dicarboxylic acid, C₁₂dicarboxylic acid, C₁₃dicarboxylic acid, C₁₄dicarboxylic acid, C₁₆dicarboxylic acid, C₁₈dicarboxylic acid, and any combinations thereof. In certain embodiments, all or part of the C_(n)dicarboxylic acids are biobased.

In another embodiments, the polyamide has a structure described above, wherein:

-   -   the polyamide is formed by reacting biobased cadaverine with one         or more dicarboxylic acids, more preferably the biobased         cadaverine is prepared according to the method disclosed herein.     -   n=4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,         21, or 22;     -   j=about 100˜about 1,000,000, about 1000˜about 100,000, or about         1000˜about 10,000; and     -   the dicarboxylic acids comprise biobased carbon under Standard         ASTM D6866.

Another aspect of the present disclosure relates to a method of preparing 1,5-diisocyanatopentane comprising:

2a) preparing biobased cadaverine as disclosed herein; and

2b) converting biobased cadaverine obtained from step 2a to 1,5-diisocyanatopentane.

Step 2b may comprise using any known method to convert diamine into isocyanate. An example of said method is the traditional phosgene method, which includes one-step high temperature phosgene method (i.e. mixing phosgene with diamine at high temperature to obtain isocyanate), the improved two-step phosgene method, and the triphosgene method in which triphosgene is used instead of phosgene. There are also other methods that do not use phosgene as a raw material. An example of said method is hexanediamine carbonylation which uses CO₂ instead of phosgene: CO₂ is added into a solution of a primary amine and an organic base, then a proper amount of phosphorus electrophilic reagents is added into the reaction solution to start an exothermic dehydration reaction to obtain isocyanate. Another example is carbamate thermal decomposition method wherein a primary amine is converted to a carbamate, and then the carbamate is heated to decompose and generate isocyanate.

The following examples are intended to illustrate various embodiments of the invention. As such, the specific embodiments discussed are not to be construed as limitations on the scope of the invention. It will be apparent to one skilled in the art that various equivalents, changes, and modifications may be made without departing from the scope of invention, and it is understood that such equivalent embodiments are to be included herein. Further, all references cited in the disclosure are hereby incorporated by reference in their entireties, as if fully set forth herein.

EXAMPLES Example 1 Construction of cadA Recombinant Expression Plasmid Vector

cadA gene was amplified with primers 1 and 2 (primer 1, SEQ ID:NO 7: ATGAACGTTATTGCAATATT, SEQ ID:NO 8: primer 2: ACTGAAAGCTTCCACTTCCCTTGTACGAGCT), using E. coli BL21 (purchased from Biomed) chromosomal DNA as template (FIG. 1 a). The PCR product was ligated to a pUC18-derived T vector, pMD18-T (TaKaRa). The ligation product that was selected contained the cadA gene and lac promoter (Plac) positioned in the same orientation. The resulting plasmid is named pMD18-T-cadA (FIG. 1 b).

pMD18-T-cadA contained a cadA gene in frame with a short lacZ fragment located at the 5′ end. Subsequently, this plasmid was subjected to nucleotide deletion via site-specific mutagenesis PCR. The PCR reaction contained: 50 ng plasmid DNA, 10 pmole primer 3 (SEQ ID:NO 9: ATTCAATATTGCAATAACGTTCATAGCTGTTTCCTGTGTG), dNTPs (0.25 mM each), 1 μL Pfu DNA polymerase (Biomed), 1 μL Taq DNA ligase (NEB), 4 μL Pfu DNA polymerase 10× buffer, 5 μL Taq DNA ligase 10× buffer and deionized water added to a total volume of 50 μL. The thermal condition was set as regular PCR. At the end of the PCR reaction, 1 μL Dpnl (NEB) was added and the reaction was incubated at 37° C. for 1 hour. 100 μL of E. coli BL21 competent cells were transformed with 10 μL of the PCR reaction. Plasmids from the transformant colonies were extracted and sequenced using primer 4 (SEQ ID:NO 10: AGGAAACAGCTATGAACGTT). The expected plasmid contained a deletion of the lacZ fragment. The resulting plasmid was named pPlac-cadA, wherein the lacZ fragment to the 5′ end of cadA gene was removed (FIG. 1 c).

The H. alvei strain used herein contained endogenous pAlvB plasmid.

The toxin/antitoxin gene pair of the endogenous pAlvB plasmid was abt/abi gene pair. Primers 5 and 6 were designed according to the published pAlvB sequence (GenBank: AY271829) to amplify a fragment containing the abt/abi genes. The primers introduced HindIII digestion sites on both ends of the fragment (primer 5: ACTGAAAGCTTTACTTTCATCACAAGCCTCT (SEQ ID:NO 11), primer 6: ACTGAAAGCTTAGATTCAGCGCGAGAGTGAT (SEQ ID:NO 12)) (FIG. 1 d). PCR was conducted with primers 5 and 6 using pAlvB as a template. The PCR product was ligated to the pMD18-T vector. The ligation product was digested with HindIII to release a fragment of about 1.8 kb containing the abt/abi genes. The pPlac-cadA plasmid was also digested with HindIII to release a fragment of about 4.8 kb. Finally, the pPlac-cadA fragment and the abt/abi fragment were ligated together to form the recombinant expression plasmid vector, pPlac-cadA-abtabi (FIG. 1 e).

FIG. 1 e shows one structure of the plasmid product for illustration purpose. One person having ordinary skill in the art would recognize, ligation of the pPlac-cadA and abt/abi fragments resulted in two types of plasmids with opposite abt/abi orientations relative to the rest of the plasmid (FIG. 2). The two orientations were identified by PCR reactions using isolated pPlac-cadA-abtabi plasmids templates. PCR reactions were conducted with either primers 1 and 5, or primers 1 and 6. The plasmid was type I plasmid (FIG. 2) when a 4 kb PCR product was produced with primers 1 and 5. The plasmid was type II plasmid (FIG. 2) when a 4 kb PCR product was producted with primers 1 and 6.

Because the H. alvei strain used herein contained endogenous pAlvB plasmid, the H. alvei strain was cured first to provide endogenous plasmid-free H. alvei strain (H. alvei ^(c)). H. alvei ^(c) strain was then transformed with the new expression plasmid (pPlac-cadA-abtabi). This new recombinant expression plasmid vector showed stability after 5 or more rounds of serial subculturing without antibiotic selection.

Example 2 Curation of the Hafnia alvei Endogenous Plasmid

A H. alvei strain having endogenous pAlvB plasmid was cured to remove the endogenous plasmid. The dependence of host survival on pAlvB was relieved by expressing recombinant antitoxin using a pUC plasmid. The pUC-derived plasmid was used as a backbone plasmid because it can replicate in H. alvei and has the ability to increase copy number upon an increase in temperature. Thus, upon antibiotic selection and temperature increase, the pUC plasmid was favorably selected and the pAlvB plasmid was lost from the cell and the recombinant Abi, overexpressed by pUC, neutralized the existing endogenous Abt toxin. As a result, the H. alvei strain survived after loss of the endogenous pAlvB rather than being killed by the endogenous Abt toxin.

The abi antitoxin gene from pAlvB was amplified using primers 6 and 7 (primer 7: ACTGAAAGCTTTTTAATTGTGTGACCACTAT (SEQ ID:NO 13)). The resulting PCR product was ligated to the pMD18-T vector (containing an ampicillin resistance gene) and was named pMD18-T-abi. The ligation product was transformed into H. alvei competent cells prepared with CaCl₂. The H. alvei competent cells were prepared the same way as E. coli. competent cells.

The transformant contained two plasmids in the cell: pAlvB and pMD18-T-abi. The transformant was streaked onto an LB/Amp plate and incubated at 40° C. overnight. Colony PCR was performed for the out-grown colonies with primers 5 and 6. Loss of pAlvB was confirmed by the lack of PCR product.

The next step was removal of the pUC plasmid from the pAlvB-cured H. alvei strain. The strain was streaked onto an LB plate with no ampicillin and incubated overnight at 40° C. An out-grown colony was restreaked on an LB plate and incubated at 40° C. overnight. Colony PCR was performed for the out-grown colonies with primers 6 and 7. Loss of pMD18-T-abi was confirmed by lack of PCR product as well as by lack of plasmid DNA after DNA extraction using a plasmid extraction kit (AxyPrep from Axygen).

The cured strain was named H. alvei ^(c) (Ha^(c)).

Example 3 Toxin/Antitoxin Gene Pair Stabilizes cadA Expression Plasmid in H. alvei

The stability of different plasmid vectors was assayed by serially subculturing recombinant strains to non-selective medium and plating the cultures on non-selective and selective plates to estimate the total cell number and the number of plasmid-containing cells.

Single colonies of three recombinant strains: JM109/pPlac-cadA, Ha/pPlac-cadA, and Ha^(c)/pPlac-cadA-abtabi (Type II), were used to inoculate LB medium containing ampicillin (JM109 is an E. coli strain; Ha denotes unmodified H. alvei containing the endogenous pAlvB plasmid; Ha^(c) denotes cured H. alvei lacking the pAlvB plasmid). The cultures were grown for 1 day at 35° C. (seed culture), and were then used to inoculate fresh LB medium without ampicillin at a rate of 0.1%. The subcultures were grown for 1 day (1^(st) subculture). Sub-culturing was continued with the same inoculation rate and the same growth conditions (2^(nd) to 5^(th) subculture). On each day, samples were taken from cultures and serially diluted with sterile 0.85% NaCl. 5 μL of diluted samples were spotted onto LB plates and LB/Amp plates. The plates were incubated for 1 day at 35° C. The total cell number and the number of cells that harbor the ampicillin resistant plasmid can be estimated from the number of colonies on LB and LB/Amp plates, respectively (FIG. 3 (JM109/pPlac-cadA), FIG. 4 (Ha/pPlac-cadA), FIG. 5 (Ha^(c)/pPlac-cadA-abtabi)).

The percentage of plasmid-bearing cells decreased to approximately 1% after 2 or 3 subcultures for the JM109/pPlac-cadA and Ha/pPlac-cadA strains (FIGS. 3 and 4, respectively). However, 100% of plasmid-bearing cells remained after 5 consecutive subcultures for strain Ha^(c)/pPlac-cadA-abtabi (FIG. 5). Thus, the toxin/antitoxin genes stabilized the recombinant expression plasmid vector in H. alvei with no need of antibiotic selection.

Example 4 Stability of Plasmid pPlac-cadA in Cured H. alvei

The stability of plasmid pPlac-cadA was assayed by culturing the recombinant strain in non-selective medium and plating the culture on non-selective and selective plates to estimate the total cell number and the number of plasmid-bearing cells.

A single colony of strain Ha^(c)/pPlac-cadA was used to inoculate LB medium containing ampicillin (Ha^(c) denotes cured H. alvei lacking the pAlvB plasmid). The culture was grown for 1 day at 35° C. (seed culture), and was then used to inoculate fresh LB medium without ampicillin at a rate of 0.1%. The subculture was grown for 1 day. Samples were taken from the seed culture and the subculture and serially diluted with sterile 0.85% NaCl. 5 μL of diluted samples were spotted onto LB plates and LB/Amp plates. The plates were incubated for 1 day at 35° C. The total cell number and the number of cells that harbor the ampicillin resistant plasmid can be estimated from the number of colonies on LB and LB/Amp plates, respectively (FIG. 6).

The plasmid was very unstable in cured H. alvei. About 0.1% of the cells retained the plasmid in the seed culture. And no plasmid-bearing cells were observed in the subculture.

Example 5 Stability of Type I pPlac-cadA-abtabi Plasmid in Cured H. alvei

The stability of type I pPlac-cadA-abtabi plasmid was assayed by serially subculturing the recombinant strain in non-selective medium and plating the culture on non-selective and selective plates to estimate the total cell number and the number of plasmid-bearing cells.

A single colony of strain Ha^(c)/pPlac-cadA-abtabi (type I) was used to inoculate LB medium containing ampicillin. The culture was grown for 1 day at 35° C. (seed culture), and was then used to inoculate fresh LB medium without ampicillin at a rate of 0.1% (1^(st) subculture). Sub-culturing was continued with the same inoculation rate and the same growth conditions (2^(nd) subculture). Both subcultures were grown for 1 day. Samples were taken from the seed culture and the subcultures and serially diluted with sterile 0.85% NaCl. 5 μL of diluted samples were spotted onto LB plates and LB/Amp plates. The plates were incubated for 1 day at 35° C. The percentage of cells that harbor the ampicillin resistant plasmid was estimated from the number of colonies on LB and LB/Amp plates (FIG. 7).

There was a significant increase in plasmid stability when abt/abi was present on the plasmid in cured H. alvei. Although there was a significant loss of plasmid in the seed culture, the plasmid was not completely lost in the subcultures like pPlac-cadA was. About 1% of the cells still had the plasmid in the second subculture.

SEQUENCE LISTINGS SEQ ID: NO 1 (aat gene) >gb|AY271828.1|:385-1717 Hafnia alvei plasmid pAlvA, complete sequence    1 ttgactttgt taaaagtcag gcataagatc aaaatactgt atatataaca atgtatttat   61 atacagtatt ttatactttt tatctaacgt cagagagggc aatattatga gtggtggaga  121 tggcaagggt cacaatagtg gagcacatga ttccggtggc agcattaatg gaacttctgg  181 gaaaggtggg ccatcaagcg gaggagcatc agataattct gggtggagtt cggaaaataa  241 cccgtggggc ggtggtaact cgggaatgat tggtggcagt caaggaggta acggagctaa  301 tcatggtggc gaaaatacat cttctaacta tgggaaagat gtatcacgcc aaatcggtga  361 tgcgatagcc agaaaggaag gcatcaatcc gaaaatattc actgggtact ttatccgttc  421 agatggatat ttgatcggaa taacgccact tgtcagtggt gatgcctttg gcgttaatct  481 tggcctgttc aataacaatc aaaatagtag tagtgaaaat aagggatgga atggaaggaa  541 tggagatggc attaaaaata gtagccaagg tggatggaag attaaaacta atgaacttac  601 ttcaaaccaa gtagctgctg ctaaatccgt tccagaacct aaaaatagta aatattataa  661 gtccatgaga gaagctagcg atgaggttat taattctaat ttaaaccaag ggcatggagt  721 tggtgaggca gctagagctg aaagagatta cagagaaaaa gtaaagaacg caatcaatga  781 taatagtccc aatgtgctac aggatgctat taaatttaca gcagattttt ataaggaagt  841 ttttaacgct tacggagaaa aagccgaaaa actagccaag ttattagctg atcaagctaa  901 aggtaaaaag atccgcaatg tagaagatgc attgaaatct tatgaaaaac acaaggctaa  961 cattaacaaa aaaatcaatg cgaaagatcg cgaagctatc gccaaggctt tggagtctat 1021 ggatgtagaa aaagccgcaa aaaatatatc caagttcagc aaaggactag gttgggttgg 1081 cccagctatc gatataactg attggtttac agaattatac aaagcagtga aaactgataa 1141 ttggagatct ctttatgtta aaactgaaac tattgcagta gggctagctg caacccatgt 1201 caccgcctta gcattcagtg ctgtcttggg tgggcctata ggtattttag gttatggttt 1261 gattatggct ggggttgggg cgttagttaa cgagacaata gttgacgagg caaataaggt 1321 cattgggatt taa SEQ ID: NO 2 (aai gene) >gb|AY271828.1|:1734-2069 Hafnia alvei plasmid pAlvA, complete sequence   1 ctatatttta gcggtcacat tttttatttc aaaacaaaca gaaagaacac caataggaat  61 tgatgtcata aaaataaaaa taaaatacaa agtcattaaa tatgtttttg gcacaccatc 121 cttaaaaaaa cctgttttcc aaaattcttt tttcgtatat ctaagcgctg ctttctctat 181 tagaaaccga gagaaaggaa atagaatagc gctagccaaa ccaaagattc tgagcgcaat 241 tattttaggt tcgtcatcac cataactggc gtaaagaata caagcagcca taaagtatcc 301 ccaaaacata ttatgtatgt aatatttcct tgtcat SEQ ID: NO 3 (abt gene) >gb|AY271829.1|:384-1566 Hafnia alvei plasmid pAlvB, complete sequence    1 atgagtggtg gagacggtaa aggtcacaat agtggagcac atgattccgg tggcagcatt   61 aatggaactt cggggaaagg tggacctgat tctggtggcg gatattggga caaccatcca  121 catattacaa tcaccggtgg acgggaagta ggtcaagggg gagctggtat caactggggt  181 ggtggttctg gtcatggtaa cggcgggggc tcagttgcca tccaagaata taacacgagt  241 aaatatccta acacgggagg atttcctcct cttggagacg ctagctggct gttaaatcct  301 ccaaaatggt cggttattga agtaaaatca gaaaactcag catggcgctc ttatattact  361 catgttcaag gtcatgttta caaattgact tttgatggta cgggtaagct cattgatacc  421 gcgtatgtta attatgaacc cagtgatgat actcgttgga gcccgcttaa aagttttaaa  481 tataataaag gaaccgctga aaaacaggtt agggatgcca ttaacaatga aaaagaagca  541 gttaaggacg ctgttaaatt tactgcagac ttctataaag aggtttttaa ggtttacgga  601 gaaaaagccg agaagctcgc taagttatta gcagatcaag ctaaaggcaa aaaggttcgc  661 aacgtagaag atgccttgaa atcttatgaa aaatataaga ctaacattaa caaaaaaatc  721 aatgcgaaag atcgcgaagc tattgctaaa gccttggagt ctatggatgt aggaaaagcc  781 gcaaaaaata tagccaagtt cagtaaagga ctaggttggg ttggccctgc tatcgatata  841 actgattggt ttacagaatt atacaaggca gtggaaactg ataattggag atctttttat  901 gttaaaactg aaactattgc agtagggcta gctgcaaccc atgttgccgc cttggcattc  961 agcgctgtct tgggtgggcc tgtaggtatt ttgggttatg gtttgattat ggctggggtt 1021 ggggcgttag ttaatgagac aatagttgac gaggcaaata aggttattgg gctttaa SEQ ID: NO 4 (abi gene) >gb|AY271829.1|:1583-1918 Hafnia alvei plasmid pAlvB, complete sequence   1 ctataattta gcggtcacat tttttatttc aaaaaaaaca gaaataacac ctataggaat  61 tgatgtcata aaaataaaaa ttaaatacaa agtcattaaa tatgtttttg gcacgccatc 121 cttaaaaaaa ccagtttccc aaaattcttt tttcgtatat ctaagcgcgg ttttctctat 181 taaaaaccga gagaaaggga ataggatagc actagccaaa ccaaagattc tgagcgcaat 241 tattttaggt tcgttatccc cataactggc gtaaagaata caaacagcca taaagtaccc 301 ccaaaacata ttatgtatat aatatttcct tgtcat SEQ ID: NO 5 (E. coli gene for lysine decarboxylase (cadA)) >gb|M76411.1|ECOCADABC:1913-4060 E.coli cadA gene, 5′ cds and cadB and cadC genes, complete cds    1 atgaacgtta ttgcaatatt gaatcacatg ggggtttatt ttaaagaaga acccatccgt   61 gaacttcatc gcgcgcttga acgtctgaac ttccagattg tttacccgaa cgaccgtgac  121 gacttattaa aactgatcga aaacaatgcg cgtctgtgcg gcgttatttt tgactgggat  181 aaatataatc tcgagctgtg cgaagaaatt agcaaaatga acgagaacct gccgttgtac  241 gcgttcgcta atacgtattc cactctcgat gtaagcctga atgacctgcg tttacagatt  301 agcttctttg aatatgcgct gggtgctgct gaagatattg ctaataagat caagcagacc  361 actgacgaat atatcaacac tattctgcct ccgctgacta aagcactgtt taaatatgtt  421 cgtgaaggta aatatacttt ctgtactcct ggtcacatgg gcggtactgc attccagaaa  481 agcccggtag gtagcctgtt ctatgatttc tttggtccga ataccatgaa atctgatatt  541 tccatttcag tatctgaact gggttctctg ctggatcaca gtggtccaca caaagaagca  601 gaacagtata tcgctcgcgt ctttaacgca gaccgcagct acatggtgac caacggtact  661 tccactgcga acaaaattgt tggtatgtac tctgctccag caggcagcac cattctgatt  721 gaccgtaact gccacaaatc gctgacccac ctgatgatga tgagcgatgt tacgccaatc  781 tatttccgcc cgacccgtaa cgcttacggt attcttggtg gtatcccaca gagtgaattc  841 cagcacgcta ccattgctaa gcgcgtgaaa gaaacaccaa acgcaacctg gccggtacat  901 gctgtaatta ccaactctac ctatgatggt ctgctgtaca acaccgactt catcaagaaa  961 acactggatg tgaaatccat ccactttgac tccgcgtggg tgccttacac caacttctca 1021 ccgatttacg aaggtaaatg cggtatgagc ggtggccgtg tagaagggaa agtgatttac 1081 gaaacccagt ccactcacaa actgctggcg gcgttctctc aggcttccat gatccacgtt 1141 aaaggtgacg taaacgaaga aacctttaac gaagcctaca tgatgcacac caccacttct 1201 ccgcactacg gtatcgtggc gtccactgaa accgctgcgg cgatgatgaa aggcaatgca 1261 ggtaagcgtc tgatcaacgg ttctattgaa cgtgcgatca aattccgtaa agagatcaaa 1321 cgtctgagaa cggaatctga tggctggttc tttgatgtat ggcagccgga tcatatcgat 1381 acgactgaat gctggccgct gcgttctgac agcacctggc acggcttcaa aaacatcgat 1441 aacgagcaca tgtatcttga cccgatcaaa gtcaccctgc tgactccggg gatggaaaaa 1501 gacggcacca tgagcgactt tggtattccg gccagcatcg tggcgaaata cctcgacgaa 1561 catggcatcg ttgttgagaa aaccggtccg tataacctgc tgttcctgtt cagcatcggt 1621 atcgataaga ccaaagcact gagcctgctg cgtgctctga ctgactttaa acgtgcgttc 1681 gacctgaacc tgcgtgtgaa aaacatgctg ccgtctctgt atcgtgaaga tcctgaattc 1741 tatgaaaaca tgcgtattca ggaactggct cagaatatcc acaaactgat tgttcaccac 1801 aatctgccgg atctgatgta tcgcgcattt gaagtgctgc cgacgatggt aatgactccg 1861 tatgctgcat tccagaaaga gctgcacggt atgaccgaag aagtttacct cgacgaaatg 1921 gtaggtcgta ttaacgccaa tatgatcctt ccgtacccgc cgggagttcc tctggtaatg 1981 ccgggtgaaa tgatcaccga agaaagccgt ccggttctgg agttcctgca gatgctgtgt 2041 gaaatcggcg ctcactatcc gggctttgaa accgatattc acggtgcata ccgtcaggct 2101 gatggccgct ataccgttaa ggtattgaaa gaagaaagca aaaaataa SEQ ID: NO 6 (Hafnia Alvei gene for lysine decarboxylase (haldc) >gi|43438|emb|X03774.1| Hafnia alvei gene for lysine decarboxylase (LDC)    1 atgaatatca ttgccatcat gaacgattta agcgcttatt ttaaggaaga acccctgcgc   61 gagctgcatc aagagttaga gaaggaaggc ttccgtattg cttatcccaa agaccgcaac  121 gatctgctga agctgattga aaacaactcc cgcctgtgtg gcgtcatttt cgactgggat  181 aaatataacc tcgaactcag cgctgaaatc agcgagctca acaaactgct gccaatttat  241 gccttcgcca atacctattc gacgcttgac gtcaacatga gcgacctgcg tcttaatgtt  301 cgcttctttg aatatgcatt aggcagcgcg caagacattg ccaccaagat ccgccaaagc  361 accgatcagt atattgatac cattctgcca ccgctgacca aggcgctgtt caaatacgtc  421 aaagaagaga aatacacagt ctgtacgccg gggcatatgg gcggaactgc gttcgataaa  481 agccctgtcg gtagcctgtt ctatgatttc ttcggtgaaa acaccatgcg ttcggatatc  541 tcgatctccg tatctgagct cggatcgctg ctcgatcata gcggcccaca ccgtgacgcc  601 gaagagtata tcgcgcgcac gttcaacgcc gatcgcagct atatcgtaac caacggaaca  661 tctacggcga ataaaattgt cggcatgtat tcatctcctg ccggtgccac tattctgata  721 gaccgtaact gccataaatc attgacccat ttgatgatga tgagcaacgt tgtccccgtc  781 tatctgcgcc caacccgtaa cgcctacggc attttaggcg ggataccgca aagcgagttc  841 acccgcgcca gcattgaaga gaaagtgaaa aatacgccca atgcgacatg gccggtgcat  901 gcggtagtca ccaactctac ctatgacggc ctgttctaca ataccgaata catcaaaaac  961 acgcttgatg ttaagtcgat tcacttcgat tcggcatggg tgccttacac caacttccat 1021 ccgatttacc aaggcaaagc agggatgagc ggtgaacgtg tgccggggaa aatcatctac 1081 gagactcagt ccacccacaa actgctggcg gcattctcgc aggcatcgat gatccacgtg 1141 aaaggtgaga tcaacgaaga aaccttcaat gaagcctata tgatgcatac ctcaacatca 1201 ccgcattacg ggatcgttgc gtcgacggaa accgcggcgg ctatgatgaa gggcaacgcc 1261 ggtaagcgtt taattaacgg ttcaattgaa cgagcgatcc gcttccgtaa agagatccgc 1321 cgcttacgta cagaatctga tggctggttc tttgacgtat ggcagccgga taacattgac 1381 gaggttgctt gctggccact caatccacgt aatgaatggc atggattccc gaacatcgac 1441 aacgatcata tgtatcttga tccgatcaaa gtcactctgc tgaccccagg tttaagcccc 1501 aatggcactc tggaagagga agggataccg gcgtcgatcg tgtcgaaata tctggatgag 1561 cacggcatca tcgtggaaaa aaccgggcca tataacctgc tcttcctgtt tagtatcggg 1621 atcgataaaa ccaaggcgtt gagcttgttg cgggcattaa ccgatttcaa acgcgtgtat 1681 gacctcaacc tgcgcgtgaa aaacgtgttg ccatcgctct ataacgaggc gcctgatttc 1741 tataaagaga tgcgaattca ggagttggct caggggattc atgctctggt gaaacaccac 1801 aatctaccag acctgatgta tcgtgcattt gaggtattac caaagctggt gatgacgccg 1861 catgatgcgt tccaagaaga agtgcgtggc aatattgagc catgtgcctt ggatgatatg 1921 ttagggaaag ttagcgccaa catgatcttg ccgtatcctc cgggtgttcc ggtggttatg 1981 ccgggagaaa tgctcactaa ggagagccgc cctgttctga gcttcttgca gatgctatgt 2041 gaaattggcg cacactatcc gggctttgaa acggatattc acggcgttca tcgtgatggt 2101 gcaacgggta aatacatggt cgtggtgctc aaacaaggcg cagatgaacc gggtgataaa 2161 ccgagtgata cggtgaagaa agcgccgggt aaaaaaccat cagcggcgaa gaagtcataa SEQ ID: NO 7    1 ATGAACGTTA TTGCAATATT SEQ ID: NO 8    1 ACTGAAAGCT TCCACTTCCC TTGTACGAGC T SEQ ID: NO 9    1 ATTCAATATT GCAATAACGT TCATAGCTGT TTCCTGTGTG SEQ ID: NO 10    1 AGGAAACAGC TATGAACGTT SEQ ID: NO 11    1 ACTGAAAGCTT TACTTTCATC ACAAGCCTCT SEQ ID: NO 12    1 ACTGAAAGCTT AGATTCAGCG CGAGAGTGAT SEQ ID: NO 13    1 ACTGAAAGCT TTTTAATTGT GTGACCACTA T 

What is claimed is:
 1. A stabilized recombinant expression plasmid vector comprising: a polynucleotide encoding a polypeptide that counteracts a toxin polypeptide toxic to a host cell, the toxin polypeptide being encoded by aat or abt gene, or mutants thereof, such that the toxin polypeptide has substantially the same function as the polypeptide encoded by aat or abt gene, in the host cell, a polynucleotide encoding the toxin polypeptide, and a polynucleotide encoding a polypeptide expression product, wherein: the stabilized recombinant expression plasmid vector is derived from a Hafnia alveiautonomously replicable backbone plasmid, and the host cell is Hafnia alvei, and the toxin gene and the antitoxin gene comprise a gene pair selected from the group consisting of aat/aai gene pair, abt/abi gene pair, and encoding functional fragments thereof.
 2. The recombinant expression plasmid vector of claim 1, wherein the toxin gene comprises a DNA sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:3, and fragments thereof.
 3. The recombinant expression plasmid vector of claim 1, wherein the antitoxin gene comprises a DNA sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4, and fragments thereof.
 4. The recombinant expression plasmid vector of claim 1, wherein the backbone plasmid is selected from the group consisting of pUC, pUC18/19, pBR322, pACYC and any derived plasmids thereof.
 5. The recombinant expression plasmid vector of claim 1, wherein the polypeptide expression product is an enzyme selected from the group consisting of decarboxylase, hydrolases and phosphorylase.
 6. The recombinant expression plasmid vector of claim 5, wherein the decarboxylase is an amino acid decarboxylase selected from the group consisting of lysine decarboxylase, tyrosine decarboxylase, arginine decarboxylase, ornithine decarboxylase, and glutamate decarboxylase.
 7. The recombinant expression plasmid vector of claim 6, wherein the polynucleotide encoding lysine decarboxylase comprises a polynucleotide selected from the group consisting of haldc gene, cadA gene, and fragments thereof.
 8. The recombinant vector of claim 7, wherein the recombinant vector comprises a DNA sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:6, and fragments thereof.
 9. The recombinant expression plasmid vector of claim 1, wherein the toxin gene comprises a DNA sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:3, and fragments thereof.
 10. The recombinant expression plasmid vector of claim 1, wherein the antitoxin gene comprises a DNA sequence selected from the group consisting of SEQ ID NO:2, SEQ ID NO:4, and fragments thereof.
 11. The recombinant expression plasmid vector of claim 1, wherein the backbone plasmid is selected from the group consisting of pUC., pUC18/19, pBR322, pACYC and any derived plasmids thereof.
 12. The recombinant expression plasmid vector of claim 1, wherein the polypeptide expression product is an enzyme selected from the group consisting of decarboxylase, hydrolases and phosphorylase.
 13. The recombinant expression plasmid vector of claim 12, wherein the decarboxylase is an amino acid decarboxylase selected from the group consisting of lysine decarboxylase, tyrosine decarboxylase, arginine decarboxylase, ornithine decarboxylase, and glutamate decarboxylase.
 14. The recombinant expression plasmid vector of claim 13, wherein the polynucleotide encoding lysine decarboxylase comprises a polynucleotide selected from the group consisting of haldc gene, cadA gene, and fragments thereof.
 15. The recombinant vector of claim 14, wherein the recombinant vector comprises a DNA sequence selected from the group consisting of SEQ ID NO:5, SEQ ID NO:6, and fragments thereof.
 16. A transformant obtained by transforming a recombinant expression plasmid vector of claim 1 into a host cell, wherein the host cell is a Hafnia alveistrain free of endogenous plasmids.
 17. The transformant of claim 16, wherein the Hafnia alveistrain is an industrial Hafnia alveistrain.
 18. A method of producing cadaverine (1,5-pentanediamine) comprising: 1a) cultivating the transformant of one of claim 11; 1b) producing cadaverine using the culture obtained from step 1a to decarboxylate lysine; and 1c) extracting and purifying cadaverine from the reaction obtained from step 1b.
 19. A method of preparing 1,5-diisocyanatopentane comprising: 2a) preparing biobased cadaverine according to the method of claims 18; and 2b) converting biobased cadaverine obtained from step 2a to 1,5-diisocyanatopentane. 